gh-36737: Fix make SPKG-uninstall for Python packages after #36452

    
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#36452 broke `make SPKG-uninstall` for Python packages.

Reproducer:
```
$ make zipp-uninstall
...
if [ -d '/Users/mkoeppe/s/sage/sage-rebasing/worktree-
clean/local/var/lib/sage/venv-python3.10' ]; then sage-spkg-uninstall
zipp '/Users/mkoeppe/s/sage/sage-rebasing/worktree-
clean/local/var/lib/sage/venv-python3.10'; fi
Uninstalling existing 'zipp'
Running pip-uninstall script for 'zipp'
ERROR: Could not open requirements file: [Errno 2] No such file or
directory: '/Users/mkoeppe/s/sage/sage-rebasing/worktree-
clean/local/var/lib/sage/venv-python3.10/var/lib/sage/scripts/zipp/spkg-
requirements.txt'
Warning: pip exited with status 0
Removing stamp file '/Users/mkoeppe/s/sage/sage-rebasing/worktree-
clean/local/var/lib/sage/venv-
python3.10/var/lib/sage/installed/zipp-3.11.0'
$ ./sage -pip list | grep zipp
zipp                           3.11.0
```



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### ⌛ Dependencies

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URL: #36737
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee

build/sage_bootstrap/uninstall.py

@@ -172,6 +172,17 @@ def modern_uninstall(spkg_name, sage_local, files, verbose=False):
         else:
             sys.exit(1)
 
+    # Run the package's piprm script, if it exists.
+    # Since #36452 the spkg-requirements.txt file appears in the installation
+    # manifest, so this step has to happen before removing the files.
+    try:
+        run_spkg_script(spkg_name, spkg_scripts, 'piprm',
+                        'pip-uninstall')
+    except Exception:
+        print("Warning: Error running the pip-uninstall script for "
+              "'{0}'; uninstallation may have left behind some files".format(
+              spkg_name), file=sys.stderr)
+
     def rmdir(dirname):
         if pth.isdir(dirname):
             if not os.listdir(dirname):
@@ -200,15 +211,6 @@ def rmdir(dirname):
         # Remove file's directory if it is now empty
         rmdir(dirname)
 
-    # Run the package's piprm script, if it exists.
-    try:
-        run_spkg_script(spkg_name, spkg_scripts, 'piprm',
-                        'pip-uninstall')
-    except Exception:
-        print("Warning: Error running the pip-uninstall script for "
-              "'{0}'; uninstallation may have left behind some files".format(
-              spkg_name), file=sys.stderr)
-
     # Run the package's postrm script, if it exists.
     # If an error occurs here print a warning, but complete the
     # uninstallation; otherwise we leave the package in a broken